Answer this question truthfully
### Question: What causes Polycythemia vera?
### Response: Polycythemia vera is a disorder of the bone marrow. It mainly causes too many red blood cells to be produced. The numbers of white blood cells and platelets may also be higher than normal.
This is a rare disorder that occurs more often in men than in women. It is not usually seen in people under age 40. The problem is often linked to a gene defect called JAK2V617F. The cause of this gene defect is unknown.